Electrical nerve stimulators have become widely used in recent years in the field of medicine for the treatment of chronic intractable pain. Such devices include circuitry for generating electrical pulses, and electrode leads for delivering the pulses to the site of pain within the body. The electrical stimulating pulses produce the effect of masking the sensation of pain, and this method is preferable to drug therapy for many types of pain, because it avoids subjecting the patient to possible dangerous side effects. In the control of chronic pain by a nerve stimulator, there are generally provided adjustments or controls so that the stimulation delivered by the device can be adjusted or controlled according to the needs of the patient, which sometimes vary from day to day, or even minute to minute. Ideally, the stimulus repetition rate, the stimulus intensity, and the stimulus situs should be controllable to provide maximum flexibility in meeting the patient's needs. Transcutaneous stimulators are worn or carried outside the body and have electrodes secured to the skin over the affected area to apply the electrical stimulation thereto. Generally, transcutaneous stimulators comprise a set of electrodes or an electrode pair with leads connected to a portable controller, with adjustments for stimulus frequency and intensity. Electrical stimulation is generally provided with pulses that may be adjusted for varying frequency, width, or amplitude. In the prior art, monophasic and bi-phasic type pulses are generally used.
Monophasic type pulses induce current in the body tissue that flows in only one direction, the electrodes having a fixed polarity. It is generally known in the art that monophasic pulses of this type produce a "stinging" sensation or pain for the patient. The bi-phasic pulse is comprised of two sequential monophasic pulses of alternating polarity. In this manner, electrical nerve stimulation may be produced without a net DC current, thus avoiding the "stinging" pain associated with it.
While the most common method of controlling stimulation intensity is varying the pulse width or amplitude, the technique of temporal integration or temporal summation is also known in the art. To achieve varying degrees of stimulation intensity using temporal summation, two or more threshold or sub-threshold stimulation pulses are delivered to the body tissue in a sufficiently short period of time so that they are summed or integrated to produce a discernible stimulus. Hereinafter these "pulse trains" will be referred to as pulse bursts. The technique is advantageous in that a series of relatively low amplitude pulses (approximately 5 milliamps or less) may be used to produce stimulation requiring 80 milliamps or more using the single pulse stimulation method. Specifically, the advantage comes from being able to use a standard FET current source as opposed to the fly back transformer or pulse transformer current source most commonly used in the single pulse method and from the reduced likelihood of the patient's experiencing "stinging pain".
In a conventional electrical nerve stimulation system, stimulation pulses are delivered to body tissue through one pair of electrodes. Generally, each electrode has an area on the order of 4.5 square inches. While this method is advantageous because of its simplicity, it is hindered by its poor ability to distribute current uniformly throughout the volume of tissue beneath and between electrode pads. The problem is generally attributed to the varying conductivity of the tissue and the tissueelectrode interface. Usually, local tissue breakdown results in a single isolated current path from one electrode to the other. Through the use of a multiple electrode system where each electrode has an area on the order of 10 square millimeters, this problem has been alleviated. However, to operate a multiple electrode system using the conventional single pulse method, it is necessary that each electrode pair have an independent current source. This is generally impractical in a portable system because of space and weight limitations. In addition to providing for better current distribution in the stimulated tissue, the multiple electrode system may be used to generate spatial patterns of stimulation. Such a prior art device exists, using switches to manually select spatially separated electrode pairs.
While pulse width and pulse amplitude provide control over perceived stimulus intensity, it is well known that controlling the pulse rate may also improve the efficacy of stimulation. Many prior art devices provide adjustable pulse rate with a manual control. While such manual control provides the patient with a means for varying the stimulus perception when any particular pulse rate becomes painful, annoying, or imperceptible, the method usually requires that the patient adjust the pulse rate regularly. This is sometimes impracticable, i.e., if the patient is sleeping or his hands are immobile. Much of the difficulty associated with manual adjustment of pulse rate has been overcome through the use of automatic controls that vary the pulse rate randomly or rhythmically, such as the 1/f fluctuation technique. Findings have shown that such methods are also more effective in alleviating pain. See Pain Control by Transcutaneous Electrical Nerve Stimulating Using Irregular Pulse of 1/f Fluctuation, Kintomo Takakura, Keiji Sano, Yukio Kosugi & Jyun Ikebe, Applied Neurophysiology, D. 42, 1979, page 314. The efficacy of electrical nerve stimulation is also improved by modulating the frequency of the pulse rate variation.
Although the automatic control of pulse rate, pulse width, and pulse amplitude has been provided for in prior art systems, an automatic system combining these features with spatial variation of the stimulus situs has not. However, due to the wide variety of patient needs, a demand exists for such a system.